A switchgear is an electric device in a power transmission system, and can be used to turn on or off, control or protect electrical power system during power generation, transmission, distribution, conversion and consumption. A main circuit of switchgear is a part in an electrical control circuit through which a high current flows, and comprises electrical components connected between a power supply and a motor. As the operating time of the switchgear increases, the resistance value of electrical joint areas (e.g. contacts) in the main circuit of switchgear may increase due to mechanical shock, abrasion and manufacturing defects, which can make the temperature of the contacts increase or even burn the contacts, resulting in security incidents.
At present, at home and abroad, by measuring the temperature of a contact in the main circuit of switchgear using a temperature sensor, the electrical connection of the contact may be determined to be abnormal when the temperature of the contact exceeds a threshold value. However, since the current in the main circuit of switchgear is not constant, the temperature of the contact in the main circuit may vary under different currents. FIG. 1 is a temperature rising diagram of electrical connectors in a main circuit of switchgear under different currents. FIG. 1 shows the temperature rising values for the electrical connectors when currents in the main circuit are 500 A, 1250 A, 1375 A and 1500 A, wherein the first to fourth data with the same current indicate the temperature rising values for a joint area of an A-phase upper bus bar, an A-phase upper contact, an A-phase lower contact and a joint area of an A-phase lower bus bar respectively, the fifth to eighth data with the same current indicate the temperature rising values for a joint area of a B-phase upper bus bar, a B-phase upper contact, a B-phase lower contact and a joint area of a B-phase lower bus bar respectively, the ninth to twelfth data with the same current indicate the temperature rising values for a joint area of a C-phase upper bus bar, a C-phase upper contact, a C-phase lower contact and a joint area of a C-phase lower bus bar respectively.
It can be seen from FIG. 1 that the greater the current in the same electrical connector is, the greater the temperature rising value is. Since the temperature rising values for the electrical connectors in the main circuit of switchgear will change when the currents in the main circuit of switchgear change, it is impossible to accurately determine whether the electrical connection in the main circuit of switchgear is abnormal, which will cause misjudgment. In order to accurately determine whether the electrical connection is abnormal, the same electrical connector should have the same threshold value under different currents.
In addition, if an electrical connector in the main circuit of switchgear is loose and when the current in the main circuit is much lower than rated current in the main circuit, the temperature rising value of the electrical connector is lower than the temperature rising value under rated current. In this case, it is impossible to accurately determine whether the electrical connection of the electrical connector is abnormal.